The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
Removal of Hydrogen Sulfide Gas and Iron in Well Water Using Pressurized Oxidation
Hydrogen sulfide gas and iron in well water, either together or alone, are known to create problems for homeowners. Obnoxious odors and tastes of hydrogen sulfide gas and rust staining of fixtures due to iron are the result of these problems. Many solutions have been proposed but there are some drawbacks. Most require the addition of chemicals that are expensive, or are too slow to react to meet household demands.
To overcome these problems, products have been developed that utilize natural processes and lessons learned from the agriculture industry, where once eliminating crop destroying insects by the use of toxic chemicals was a common practice. Later it was found that Mother Nature takes care of the problem without the use of chemicals.
Taking a cue from the agriculture industry, a new process has been developed to apply Mother Nature’s technique to the water industry to treat well water containing hydrogen sulfide gas and iron–-supersaturating the water with air under high pressure. This method is referred to as pressurized oxidation. This differs from the present air induction systems by providing air under high pressure.
In nature, hydrogen sulfide (H2S) gas and oxygen (O2) can’t coexist without a chemical reaction. The natural chemical reaction is H2S + O2 = water (H2O) + elemental sulfur. Figure 1 depicts the chemical reaction.
Because hydrogen sulfide gas exists in the water, it is necessary to dissolve the air into the water in order for the natural reaction to take place. It has been determined by calculation and verified by tests that air can be dissolved up to 10x under pressure as great as air being dissolved at atmospheric pressure. This is the main reason why the reaction time with pressurized air is faster. The various quantities of dissolved air at various pressures and temperatures are shown in Figure 2.
In the pressurized oxidation system, air is introduced into the water by means of an inductor. The system pressure is boosted for optimum air adsorption. The water acts like a sponge and absorbs the high pressure air. The high pressure water containing absorbed air and hydrogen sulfide gas begin to react. Because the reaction time is not immediate and is directly dependent on the ppm of the hydrogen sulfide (Figure 3), contact volume is required. The total contact volume is dependent upon the demand and hydrogen sulfide gas in ppm of the system.
The inductor is capable of producing more air than it is required. In the first treatment tank, an air release volume control exhausts the excessive air and maintains a defined water level. Because the reaction takes place in the water, the gas that remains is either nitrogen or oxygen. This entrained gas becomes liberated from the water at the faucet in the form of tiny bubbles. All hydrogen sulfide gas has reacted while in solution; thus removing all obnoxious sulfur odors. The elemental sulfur falls to the bottom of the treatment tank and can be removed periodically via a drain.
An important feature of the pressurized oxidation system is that when the water leaves the treatment tank, it continues to contain residual levels of oxygen. If the system water demand for whatever reason was to be increased to a level greater than anticipated and hydrogen sulfide is detected at the faucet, by just turning off the demand for a few minutes, the odor will dissipate. This is due to the fact that the water, right up to the faucet, has sufficient oxygen to react with the hydrogen sulfide gas. What is lacking is contact time; thus, the entire piping system downstream from the treatment tank, becomes contact volume. This is a major advantage over chemical treatment because when the system is overrun, the complete volume of the system downstream from the treatment source is not treatable. Homeowners using the untreated water become discouraged and dissatisfied.
There are several existing methods on the market used to remove both soluble, ferrous and insoluble, ferric iron from the water. For example, sand/multimedia filtration, is used to remove insoluble iron but it can’t remove the soluble iron unless a potassium permanganate or other oxidant chemical is used as a regenerant. Ion exchange softeners also can be used if the water requires softening along with iron removal. However, this type of iron removal requires chemicals and can only effectively remove ferrous iron in relatively low concentrations, therefore, ferric iron will result in fouling.
Oxidation has been determined effective in eliminating both forms of iron. There are three methods of oxidation: chlorination, ozonation and aeration. Of the three, aeration is the most economical and requires no chemicals. Atmospheric aeration due to the low adsorption rate requires large retention tanks and result in high capital costs. Thus, the most effective and economical method to eliminate both forms of iron is through the use of pressurized oxidation. The same pressurized oxidation system used to remove hydrogen sulfide gas can be utilized to treat both forms of iron. When the iron concentration exceeds 1 ppm, media filter tanks are to be added with a treated water backwash automatically programmed into the system (untreated water will result in less efficient removal of iron). Unlike hydrogen sulfide treatment, the pH of the water for effective iron removal must be between 7 and 9. By supersaturating the water with pressurized air, some of the dissolved iron (ferrous) becomes insoluble (ferric). The media in the filter tanks acts as an insoluble catalyst to enhance the remaining dissolved oxygen and turn the soluble iron into insoluble that can be trapped by the fine filtering characteristics of the media. Figure 4 (page 12) depicts the process of iron removal using the combination of pressurized oxidation and media filter tanks.
Pressurized oxidation systems, when used with media filtration and backwashed daily with treated water, provide clear ferrous and precipitated ferric iron removal treatment up to 40 ppm and extends the life of the filter media. Figure 5 depicts an example of a typical system. The pre-piped, pre-wired and pre-programmed controller, allows one valve at a time to sequentially operate and daily backwash treated water from the remaining two valves to drain.
Although the pressurized oxidization system was initially developed for hydrogen sulfide removal, it has been easily converted to an iron removal system with the addition of the media filters. The addition of the pre-programmed valves for daily backwashing allows for high ppm of iron removal and the extension of the media life. Further modular features can be added to solve additional problems. Limits are only set by the imagination.